This invention relates to the field of preparing carbon fibers from carbonaceous pitches. A typical process for manufacturing pitch based carbon fibers may include the following steps: (1) preparing a suitable pitch for spinning; (2) spinning the pitch into as-spun pitch fibers; (3) thermosetting (stabilizing) the pitch fibers to render them infusible, i.e. unmeltable; and, (4) carbonizing the fibers by heating the stabilized fibers to carbonization temperatures.
In the described process, the as-spun pitch fiber of step (2) is a thermoplastic material. Thus, additional heating of the fiber results in melting and loss of fiber structure. Therefore, prior to carbonization, the fiber must be rendered unmeltable, i.e. thermoset. The thermosetting process is commonly known as oxidative stabilization due to the heating of the fiber in the presence of an oxidizing agent. Typical stabilization processes expose the as-spun fibers to a high concentration of oxidizing agent at an initial process temperature lower than the fiber""s spinning temperature.
The stabilization process involves temperature dependent diffusion of oxygen into the fiber where the oxygen reacts with and promotes cross-linking of the pitch molecules. Because the reaction rate is temperature dependent, lower stabilization temperatures require longer times to complete the oxidative stabilization of the fiber. The total oxygen required for stabilization will depend on the nature of the pitch. Generally, low softening point pitches require long periods of time and more oxygen to complete the stabilization process. Typically, the oxidizing agent is air (approximately 21% oxygen).
To improve operating economics, one would prefer to stabilize (thermoset) the as-spun fiber at high temperatures under high oxygen concentrations in order to complete the stabilization process in the shortest period of time. Unfortunately, high oxygen concentrations and elevated temperatures increase the possibility of uncontrolled exothermic oxidation reactions. Reactions of this type are particularly hazardous when highly volatile hydrocarbons are present. Most current art practices minimize the risk of thermal runaway by limiting the processing temperature and quantity of exposed fiber.
In addition to the need to prevent an uncontrolled exothermic reaction and loss of carbon mass, the stabilization process must also preserve the structure of the fiber. Accordingly, the heating temperature must not exceed the fiber""s softening point. Therefore, fibers prepared from soft, low melting pitches must be stabilized at lower temperatures than fibers prepared from hard, high melting pitches.
Clearly, when treating a large amount of fiber over a short period of time the current manufacturing methods have significant drawbacks.. The need to limit temperature, oxidant concentration and quantity of fiber in the stabilization process creates higher than desirable costs, diminishes the value and strength of the fiber and creates obvious operating risks. In overcoming the deficiencies of the current processes, a preferred method would utilize a low concentration of oxidizing agent coupled with high temperature heating while avoiding the risk of thermal runaway and loss of fiber size. Preferably, such a method would yield stabilized fibers in a short period of time and generate increased operating efficiencies.
To achieve these goals, the present invention provides a process for stabilizing pitch fibers using low concentrations of oxidizing agent at high temperature in a short period of time. This novel process stabilizes the core of the fiber without excessive surface oxidation. Additionally, the current invention provides a pitch fiber which becomes stabilized at its core at a rate which is sufficient to preclude excess loss of carbon at the fiber""s surface due to oxidation. Further, the fibers take up a minimal amount of oxygen. These and other benefits of the present invention are described in greater detail below. For the purposes of this disclosure, the terms xe2x80x9cstabilizingxe2x80x9d and xe2x80x9cthermosettingxe2x80x9d are used interchangeably.
The present invention provides a novel process for stabilizing pitch fibers. According to the disclosed process, the pitch fibers are heated at a temperature equal to or greater than the spinning temperature of the fibers. During the heating process, the fibers are exposed to an oxidizing agent for a period of time sufficient to stabilize, i.e. thermoset, the fibers.
Additionally, the present invention provides a process for stabilizing pitch fibers using continuous heating in the presence of a stream of gas. This process provides a means of significantly reducing the risk of uncontrolled exothermic reactions. According to this novel process, the pitch fibers are heated to a temperature at least equal to the spinning temperature of the fibers. During the heating process, the fibers are contacted with a flowing gas which contains an oxidizing agent. The flow rate of the gas is sufficient to remove excess heat from the fibers during the stabilization process thereby controlling the exotherm of the reaction. Exposure of the fibers to the oxidizing agent is maintained for a period of time sufficient to stabilize the fibers.
Further, the present invention provides a pitch fiber having a softening point of at least 300xc2x0 C. The novel fiber has an oxygen diffusion rate to its center which is approximately equal to, or greater than, the oxidation rate at the fiber""s surface. Thus, the fiber""s center becomes oxidatively stabilized at a rate ranging from slightly less than, to greater than the rate of consumption of carbon by oxygen at the fiber""s surface. In this manner, the current invention precludes excess loss of carbon at the surface of the fiber. Oxidative stabilization of the fiber may be carried out at temperatures equal to or greater than the fiber""s spinning temperature in an atmosphere containing up to ten percent oxidizing agent by volume. Preferably, the concentration of oxidizing agent will be less than eight percent by volume. Finally, depending upon operating conditions and raw material used, these fibers may be oxidatively stabilized in less than ten minutes.
The current invention additionally provides a pitch fiber batt having a density of at least 900 g/m2 which is capable of being oxidatively stabilized. Despite the high density of fibers, the novel pitch fiber batt oxidatively stabilizes without loss of fiber structure when heated in a flowing gas stream containing an oxidizing agent.
The following discussion will focus on the stabilization of pitch fibers. However, the current invention is equally applicable to the stabilization of other artifacts prepared from pitch.
A. High Temperature Stabilization of Pitch Fibers
The stabilization of pitch fibers is a process which cross-links the large aromatic molecules of the pitch. Oxygen also reacts with pitch carbon to form gaseous carbon oxides in a process known as burnoff. If diffusion is relatively slow, oxidation at the surface (burnoff) dominates while the fiber""s center remains unstabilized. If diffusion is relatively fast, oxygen penetrates and stabilizes (cross-links) the interior of the pitch artifact with little surface burnoff. According to the current invention, the oxygen diffusion rate into the pitch fiber to effect stabilization must be comparable to or faster than the rate at which oxygen reacts to consume carbon at the fiber""s surface. Thus, the fibers may be stabilized at process temperatures of 300xc2x0 C. and above.
Prior to the current invention, those skilled in the art believed that stabilization conditions of high temperature and low oxygen concentrations would produce excessive burnoff of the fiber""s surface due to insufficient oxygen diffusion to the center of the fiber. Ultimately, the burnoff would weaken or destroy the fiber. As discussed above, increasing the concentration of oxygen at high temperatures as a means of increasing the reaction rate is not an option due to the risks of fiber melting and excessive exothermic reactions. In spite of the teachings of the prior art, the examples provided below clearly demonstrate that the present invention provides a process for stabilizing pitch fibers at high temperatures and low concentrations of an oxidizing agent.
In the preferred embodiment of the current invention, the oxidizing agent is oxygen at a concentration of eight percent (8%) by volume in a carrier gas. The preferred carrier gas is nitrogen. This novel process utilizes pitch fibers which have softening points in excess of 300xc2x0 C. These fibers may be prepared by spinning solvated mesophase pitch followed by removal of the solvating solvent from the as-spun pitch fibers. The process of preparing solvated mesophase pitch is disclosed in U.S. Pat. Nos. 5,259,947; 5,437,780 and 5,540,903 incorporated herein by reference. Further, the preparation of fibers from solvated mesophase pitch is discussed in U.S. patent application Ser. No. 08/791,443, now U.S. Pat. No. 5,766,523 and U.S. Pat. No. 5,648,041 incorporated herein by reference.
In the current process, fibers are prepared by spinning solvated mesophase pitch at a temperature in the range of 220xc2x0 C. to 340xc2x0 C. Following spinning of the fibers, the solvating solvent is removed from the as-spun pitch fibers. Typically, the solvent is removed by evaporation aided by heating and exposure of the fiber to a flowing gas. However, the method of removing the solvent is not critical to the current invention. Removal of the solvent increases the softening point of the fibers by at least 400xc2x0 C. Frequently, removal of the solvent will raise the softening point of the fiber by 100xc2x0 C. or more.
In the preferred embodiment of the current invention, solid pitch fiber is rapidly heated to an initial process temperature. The initial process temperature is greater than the spinning temperature of the fiber; yet, lower than the softening point of the pitch prior to solvation (dry pitch). The initial process temperature may range from 100xc2x0 to 900xc2x0 C. below the softening point of the dry pitch. Preferably, the initial process temperature is at least 400xc2x0 C. below the softening point of the dry pitch. Accordingly, the initial process temperature may range from 250xc2x0 C. to 500xc2x0 C. with a preferred initial process temperature of at least 300xc2x0 C.
In general, the fibers are heated at a rate sufficient to reach the initial process temperature in less than 15 minutes and preferably less than 5 minutes. To effect stabilization, the present invention maintains the initial process temperature for 1 to 60 minutes. Following this initial time period, the temperature may be increased if additional stabilization is required; however, the process temperature must be maintained below the fiber""s instantaneous softening point. Total stabilization time will depend on a number of factors including fiber melting temperature, fiber diameter, oxidant concentration and oxidation temperature. Typically, the total processing time will range from about 1 to about 150 minutes. Preferably, the total heating time is less than 60 minutes. More preferably, the total heating time will be less than 10 minutes.
During the described heating process, a flowing gas stream containing an oxidizing agent contacts the fibers. The concentration of oxidizing agent may range from approximately 2% by volume to nearly 21%. Preferably, the concentration of oxidizing agent will be less than 10% by volume. In general, the process of the present invention utilizes oxygen as the oxidizing agent and nitrogen as the carrier gas. However, other oxidizing agents and gases will function within the scope of the current invention. For example, mild oxidizing gases such as oxides of nitrogen, oxides of sulfur, carbon dioxide, chlorine, or mixtures thereof with or without a carrier gas will also function within the scope of the current invention.
The gas stream described above serves two purposes. First, it carries the oxidizing agent into contact with the pitch fibers. Second, passage of the gas stream through the fibers removes excess heat from the fibers. Thus, the present invention allows one to control the exothermic reaction inherent in the stabilization process by varying the flow rate of the gas, the concentration of oxygen and the density of the fiber batt. Preferably, these variables will be balanced such that the exothermic reaction will increase temperatures by less than 50xc2x0 C. In this manner, the present invention significantly reduces the risk of uncontrolled thermal reactions.
The following examples are intended to aid in an understanding of the current invention and are not considered limiting of the scope of the invention. In the following examples, complete stabilization is determined by exposing the fibers to the open flame of a match until the fibers become incandescent. Fibers are deemed fully stabilized if they do not melt during the xe2x80x9cmatch testxe2x80x9d. Volumes indicated in the following examples are considered to be measured at standard temperature and pressure.